The present invention relates to a brushless motor including a rotor having magnetic pole portions arranged such that all the polarities are the same, and an iron core portion, which is located between circumferentially adjacent magnetic pole portions and permits the magnetic flux of the magnetic pole portions to pass in the radial direction.
In a conventional brushless motor including a rotor having magnetic pole portions formed by permanent magnets, it has been proposed to increase motor torque by increasing the number of the magnetic poles, or by forming the magnetic pole portions with strong permanent magnets.
However, strong permanent magnets such as neodymium magnets are expensive, and there are limitations in reducing the thickness of the magnets. Thus, the manufacturing costs are increased when manufacturing a multipole motor (for example, an 8-pole 12-slot motor) using the strong permanent magnets as disclosed in Japanese Laid-Open Patent Publication No. 2008-113531.
Japanese Laid-Open Patent Publication No. 10-150755 proposes a brushless motor including a consequent pole rotor in which the magnetic pole portions formed in the rotor are arranged such that the polarities of all the magnetic pole portions are the same pole, and an iron core portion is provided between circumferentially adjacent magnetic pole portions to permit the magnetic flux from the magnetic pole portions to pass along the radial direction of the rotor.
More specifically, four magnetic pole portions 33 are provided on a rotor 32 of a brushless motor 31 at equal intervals in the circumferential direction as shown in FIG. 42. The polarities of all the magnetic pole portions 33 are the same, and all the magnetic pole portions 33 are accommodated in magnet accommodating holes H such that the radially outer sides are south poles in the rotor 32 shown in FIG. 42. The magnetic pole portions 33 are formed by plate-like permanent magnets 30. Also, gaps 34 are provided on both ends of each magnetic pole portion 33 in the circumferential direction. The gaps 34 function as magnetic resistance. An iron core portion 36 is formed between circumferentially adjacent magnetic pole portions 33. The iron core portion 36 is magnetically divided from the magnetic pole portions 33 in the circumferential direction.
As shown in FIG. 43, the magnetic flux of the pole portions 33 flows into the corresponding iron core portion 36 via the inner part of the rotor 32 bypassing the gaps 34 formed on the circumferential ends of the magnetic pole portions 33. As the magnetic flux passes through each iron core portion 36 in the radial direction, a pseudo magnetic pole having different polarity from the magnetic pole portions 33 that are circumferentially adjacent to the iron core portion 36 is formed in the iron core portion 36. The pseudo magnetic poles shown in FIG. 42 are parts shown by areas α, and radially outer sides are north poles.
That is, such a consequent pole rotor 32 reduces the number of the permanent magnets 30 by half as compared to a rotor 42 of a normal brushless motor 41 in which the permanent magnets 30 are arranged such that the polarities of circumferentially adjacent magnetic poles (magnetic pole portions 43) are different from each other as shown in FIG. 44. Thus, the consequent pole rotor 32 achieves the same advantages as a multipole rotor without increasing the manufacturing costs.
However, since a magnetic field formed in the stator, which is provided radially outward of the rotor, acts on the rotor during activation of the motor, the magnetic flux that passes in the radial direction is increased at some part and decreased at other part in the iron core portions forming the pseudo magnetic poles as described above. Thus, the magnetic center position in each iron core portion is shifted, and as a result, the magnetic balance of the rotor might fluctuate.
That is, normally, when one of the magnetic poles provided on the rotor is at a position that faces two teeth in the brushless motor, a magnetic field having magnetic force that attracts the magnetic pole is formed in the leading tooth among the two teeth, and a magnetic field having magnetic force that repels the magnetic pole is formed in the trailing tooth.
Here, in the normal brushless motor 41 having the rotor 42 as shown in FIG. 45, the magnetic flux formed by the permanent magnets 30 is not partially reduced or increased by the magnetic field of a stator 35. Thus, the magnetic balance of the rotor 42 does not fluctuate since the magnetic center position of the magnetic poles in the state of FIG. 45, that is, the position where a straight line N0 passes in FIG. 45 is not significantly shifted.
However, in the case with the brushless motor 31 including the consequent pole rotor 32 as shown in FIG. 46, when one of the iron core portions 36 forming the pseudo magnetic pole is at the position to face two teeth 37 (37a, 37b), the magnetic center position is significantly shifted forward in the rotation direction of the rotor 32, that is, to the position where a straight line N1 passes in FIG. 46.
That is, while the magnetic flux easily flows by the magnetic attractive force formed by the tooth 37a and a coil 38 at part of the iron core portion 36 that faces the leading tooth 37a, the flow of the magnetic flux is hindered by the magnetic repulsive force formed by the tooth 37b and the coil 38 at part of the iron core portion 36 that faces the trailing tooth 37b. 
That is, while the magnetic field of the stator 35 draws out the magnetic flux that passes through the iron core portion 36 in the radial direction at the leading section of the iron core portion 36, the magnetic field of the stator 35 hinders the magnetic flux from passing in the radial direction at the trailing section of the iron core portion 36. As a result, the magnetic center position in the iron core portion 36 is shifted forward of the rotation. This causes fluctuation of the magnetic balance of the rotor 32, which reduces the output of the motor, or causes noise and vibration. In this respect, there is still room for improvement.
Also, when inserting the permanent magnets 30 in the magnet accommodating holes H in the rotor 32 of the brushless motor 31 as shown in FIG. 46, the magnet accommodating portions H and the permanent magnets 30 are formed taking into consideration the dimensional tolerance of the magnet accommodating holes H and the dimensional tolerance of the permanent magnets 30 to facilitate the insertion. Thus, a gap is easily generated between the permanent magnets 30 and the magnet accommodating holes H. In particular, the gap in the radial direction reduces permeance (reciprocal of the magnetic resistance), and the magnetic flux generated in the permanent magnet is not effectively used. This reduces the output performance.